This invention relates to heat bonding plastic films, and particularly to heat bonding biaxially oriented plastic films.
Biaxially oriented plastic films are stretched in both planar axes during manufacture to re-orient their molecular structure to increase tensile strength, toughness and flexibility. When the temperature of a biaxially oriented film is increased above a given temperature, e.g. for heat bonding, the film shrinks to its pre-oriented molecular configuration, unless restrained.
To utilize biaxially oriented films, e.g. MYLAR.RTM. polyester film, in heat-sealed articles, it has previously been known to laminate such films to a layer of a non-shrinking, easily heat sealable polymer, e.g. polyethylene. To heat seal laminated films, e.g. in manufacture of the popular MYLAR.RTM. novelty balloons, the polyethylene layers are placed in contact and the laminated films are locally heated to a temperature above the bonding temperature of the polyethylene, but below the level at which the biaxially oriented film will shrink.
With the foregoing in mind, it is an object of the invention to provide an apparatus and process for producing a sealed article by directly heat bonding two or more layers of biaxially oriented polymeric film.
It is another object to join two or more layers of biaxially oriented polymeric film by heat bonding to form a seal between the layers, comprising a smooth, solid homogeneous piece of plastic having complete molecular interlocking between layers of film.
It is still another object of the invention to provide a bead seal superior to classical bar or edge seals used in the plastic film packing industry, by allowing the manufacturer to operate with higher production speeds while providing more reliable quality control, better hermetic seals and less scrap film.